1. Field of Invention
This invention relates to a leach resistant CHLOROSTAIN ink composition. The invention also relates to the use of the leach resistant ink as a security ink, i.e., for application to a document to prevent alteration of information on the document. The invention further relates to a method of forming a document having the security ink thereon.
2. Description of The Prior Art
CHLOROSTAIN-based security inks are initially colorless inks which are printed on a document, for example in the word "VOID". If bleach or some other oxidizing agent is used in an attempt to alter the document, the colorless ink forms a colored image in the shapes or letters printed on the paper.
CHLOROSTAIN-based security inks suffer from the problem that unimaged CHLOROSTAIN can be easily washed out with water. Some government jurisdictions have begun requiring that CHLOROSTAIN security features be water resistant in the unimaged state for up to one hour.
The use of benzothiazole compounds such as CHLOROSTAIN OR (Mobay Corp. Pittsburgh, Pa.) in ink compositions to prevent document alteration is known. For example, U.S. Pat. No. 4,227,719 to McElligott et al. discloses a CHLOROSTAIN OR-based security ink. However, McElligott et al., like other known CHLOROSTAIN-based security inks, suffers from the problem that the ink is not water resistant and can be washed out upon contact with water.
Recently, a security ink using N-N-diphenylguanidine (Tradename: CHLOROSTAIN BR, by Mobay Corp.) has been disclosed that provides improved water resistance. U.S. Pat. No. 5,085,925 to Dotson discloses a leach resistant security ink that includes CHLOROSTAIN BR. However, the security ink disclosed by Dotson suffers from several drawbacks. The ink composition requires the use of a complexing agent, such as glucose, sucrose, sorbitol and the like, to provide resistance to leaching. The ink disclosed in Dotson further requires the use of a separate fixing agent and fixing step to bind the ink composition to the document substrate. The separate fixing step requires either two passes through a printing or coating head or two units on the press or coater for this ink alone while the ink of the present invention only requires one print station. Further, the carrier liquid of Dotson is either alcohol, which is flammable, and therefore potentially dangerous, or water, which may distort the paper and will require drying. Further, the two step Dotson operation will either limit the other printing that can be done in one pass or will raise the cost if another printing or coating unit is added or if the paper is run through twice.
An object of the present invention is to provide a security composition that can be applied to a document to prevent unauthorized alteration of the document by the use of oxidizing agents.
Another object of the present invention is to provide an ink composition that, while initially colorless, changes color upon contact with oxidizing agents and additionally, can not be easily leached out with water.
Another object of the invention is to provide a leach resistant ink composition which does not require the use of a complexing agent.
A further object of the invention is to provide a document protected against alteration by use of a security ink composition, which composition can be applied to the document without a fixing agent or a fixing step.